Photo-masks used in the manufacturing of integrated circuits (IC) are composed of a Chrome layer deposited on Quartz or fused Silica plates, which are subsequently patterned for use as a “negative”, similarly to the printing process in photography, in a photolithographic process.
In a photolithographic process, UV (ultraviolet) light passes through the pattern inscribed on the Chrome layer, and an image is formed within the photo-resist layer on top of a Silicone wafer.
Additional layers, like protective layers, antireflective layers, or an embedded phase-shifting layer, such as MoSi, occasionally accompany the chrome layer.
Additional applications similar to IC are the lithographic processes for thin film magnetic read/write heads used in hard disk drives for data storage.
Front-end IC processes require sub-micron resolution, typically with an optical demagnification factor of 1:4 from a mask object to the printed image on Silicone wafers.
Such sub-micron processes require that critical dimensions (CD) of the features printed on the wafer, be uniform, with strict specifications.
However, it is well known throughout the semiconductor industry, that the lithography process suffers from CD variations, which often exceed specifications and requirements.
Not all CD variations are inherent to the homogeneity of the patterning on the photomask itself. In fact, a large proportion of CD variations may be attributed to external causes, such as the optical set-up used in the photolythographic process. There are various sources for CD variations, some of which are lenses aberrations, masks non-uniform patterning, illumination design, Photo-resist coating and development, etching processes and others.
Thorough studies of CD variations (Improvement of Shot uniformity on a wafer by controlling backside transmittance distribution of a photomask, Jong Rak Park, Soon Ho Kim, Gi-Sung Yeo, Sung-Woon Choi, Won-Tai Ki, Hee-Sun Yoon, Jung-Min Sohn, Samsung electronics co. LTD, SPIE proceedings, February 2003 (5040-45)) have shown that variations are in most cases, segmented to specific areas of the exposure field. An exposure field is equivalent to one projection of the mask on the wafer, hence a single wafer contains many exposure fields of the same mask. A good statistical model can specify numbers of percent of CD deviations for each area.
CD variations may be improved by taking an advantage of the fact that photo-resist threshold for activation, varies linearly with the logarithm of the exposure dose, with a slope constant—γ (“Resolution enhancement techniques in optical lithography” (chapter 1.3.4), Alfred Kwok-Kit Wong (SPIE PRESS 2001)).
If variations are a few percent above a specified value, a reduction of the UV radiation dose, will change the CD value, such as the printed line width (or contact-holes diameter) and will bring it closer to the required value.
One possible way of applying a dose reduction pattern to a photomask, on its back-side, is by patterning grooves or holes on the back-side surface of the mask (see US 20040067422, to Park et al.).
However, such a method is limited in its dynamic range, the process is slow, and suffers from high equipment cost.
There are other methods for CD control, which are commonly used in the semiconductors industry (like the focus/exposure (dose) process-window optimization, which determines the average CD on wafers) but are generally not suitable for controlling CD variations within and across the exposure fields on the wafer (Intra field CD variations).
It is an object of the present invention to provide a way that greatly reduces intra field CD variations associated with a photomask in the photolithographic process.
Yet another object of the present invention is to provide such a way for reducing CD variations that employs shading elements or diffractive optical elements embedded in the photomask.